1. Field of the Invention
This invention is generally directed to the lift arm(s) and tilt linkage mechanisms by way of which implements are raised, lowered and angularly adjusted relative to load lifting vehicles such as front-end loaders, backhoes, skid-steered tractors, forklifts and the like and more specifically to a multiple parallelogram tilt linkage system which in cooperation with the lift arm or arms of the vehicle automatically ensures that the pitch of the implement is not altered as the lift arms raise and lower the implement unless the controls for adjusting the pitch of the implement are activated. The invention is also directed to lift arm and tilt linkage systems for load elevating vehicles wherein the lift and tilt cylinders for both lifting the implement and adjusting the pitch of the implement relative to the vehicle are mounted remote with respect to the implement and are preferably directly mounted to the vehicle to thereby eliminate the need to extend hydraulic lines outwardly with respect to the vehicle where such lines would be subject to wear and tear, dirt and accidental damage.
2. History of the Related Art
One of the primary problems associated with construction and load lifting and transporting vehicles such as forklifts, bulldozers, front-end loaders, skid-steered tractors, backhoes, and the like are that the implements associated therewith are continuously subject to change in pitch relative to the vehicle as the implements are raised and lowered by the lift arm(s) of the vehicle. Due to the change in the pitch of an implement the loads supported thereby are not maintained absolutely level as they are raised and lowered. In those instances where the implement is a bucket having aggregate material contained therein, if the bucket changes from a level position the material is accidentally discharged over the edge of the bucket. Similar problems occur when transporting pallets on which materials are stacked or when transporting and aligning loads of sensitive materials such as in the lifting of munitions from ammunition carrying carts to aircraft to which the munitions are to be mounted.
In view of the foregoing problem there have been numerous attempts to provide lift arm and tilt linkage mechanisms which will compensate for the change in pitch of an implement as the implement is raised or lowered. Unfortunately, such conventional systems do not provide a true self levelling system which will ensure a continuous fixed pitch alignment between the implement and the vehicle regardless of the degree of elevation of the implement with respect to the vehicle as the implement is raised or lowered by the vehicles lift arm assembly.
In U.S. Pat. No. 4,355,946 to Wyckhuis et al. a lift arm and control linkage assembly for loading buckets is disclosed for maintaining the buckets in substantially level positions during lifting. However, as the patent indicates, the position of the bucket is only substantially maintained in a fixed pitch or position and actually the pitch changes as the bucket is raised or lowered relative to the vehicle. The system utilizes counter-clockwise pivotable linkages which are connected through a bell crank assembly to lift arms and tilt cylinders which are mounted between the bucket and the bell crank arrangement. Such a system requires that the hydraulic cylinder which controls the tilt of the bucket to be mounted to the bucket or bucket support and the bell crank mechanism at a location remote from the vehicle itself. This requires that hydraulic lines must be extended from the vehicle outwardly to the tilt cylinder(s). Such an arrangement has a disadvantage of requiring the exposure of the hydraulic cylinder outwardly of the vehicle at which point it is subject to wear and tear, dirt and other debris and also requires that hydraulic lines be extended through the lift arms thereby exposing the hydraulic lines to possible accidental damage. Therefore, not only does this linkage mechanism not provide a continuously self-levelling effect but also exposes the tilt cylinders and hydraulic lines to additional wear and tear. Similar type lift and tilt assemblies are disclosed in U.S. Pat. No. 3,237,795 to Kromer, U.S. Pat. No. 3,722,724 to Blakely, U.S. Pat No. 4,825,568 to Kawamura et al. and U.S. Pat. No. 4,264,264 to McMillan et al.
In an effort to improve the self-levelling of lift and tilt control mechanisms for vehicles having lift implements such as buckets, complicated control systems were developed to automatically adjust the hydraulics electronically in order to continuously adjust the bucket to maintain a substantially constant pitch of the bucket with respect to the vehicle as the implement raised and lowered. One such example of an electronic bucket positioning and control system is disclosed in U.S. Pat. No. 4,844,685 to Sagaser. Such systems however do not maintain a bucket in a constant or fixed pitch a minute adjustments are continuously made to the tilt cylinder linkage in order to adjust the bucket relative to the lift assembly to maintain a substantially constant pitch as the lift arms of the vehicle are raised and lowered. Such systems are also very costly in that they require the electronic features to be connected to the hydraulic control circuits to maintain the required amount of levelling. A further variation of the prior art is disclosed in U.S. Pat. No. 4,923,362 to Fryk which discloses the use a hydraulic valve system connected to an electronic control circuit for adjusting the hydraulic controls of the lift and tilt arms to maintain a bucket in a level configuration. Such a system also is expensive, complicated and does not provide a truly continuous self-levelling of an implement carried by the vehicle.
Other types of self-levelling systems have been proposed which are generally mechanical in nature. The most predominant of these appear to be referred to as "parallelogram" linkages or linkage system. An example of such a system is disclosed in British Patent 866,619 of Apr. 26, 1961. This patent discloses the use of a pair of link members which extend generally parallel to the lift arms of the implement support frame and which are pivotably connected with respect to one another by an intermediate triangular frame member with the pivots of the frame member being generally parallel to the vehicle and the pivot points on the implement support frame. In this manner, two end to end parallelograms are formed by the links and the lift arms of the vehicle. Such an arrangement does provide for a limited vertical lifting of the implement without changing pitch of the implement support frame. However with this type of arrangement, the tilt cylinder is mounted on the implement support frame and therefore any adjustment with respect to the implement must be accomplished utilizing the hydraulic cylinder which is mounted exteriorly of the vehicle on the implement support frame thereby necessitating that hydraulic lines are extended along the length of the lift arms and implement support extension members. Additional examples of parallel linkage systems are disclosed in German Offenlegungsschraft 28 22 050 of Nov. 22, 1979, German Patentscrift DD 247 643 A of Jul. 15, 1987, Soviet Union Patents 1073-087-A of Feb. 15, 1984 and 1161-372-A of Jun. 15, 1985, and U.S. Pat. No. 2,665,017 to McNamara, Jr., U.S. Pat. No. 3,703,968 to Uhrich et al., U.S. Pat No. 3,792,786 to Gokhburg et al. and U.S. Pat No. 4,583,907 to Wimberley.
Additional examples of hydraulic linkage mechanisms for supporting implements and controlling their pitch relative to lift vehicles are disclosed in U.S. Pat. No. 2,455,474 to Drott et al., U.S. Pat. No. 2,720,990 to Beyerstedt et al., U.S. Pat No. 3,175,711 to Granryd, U.S. Pat. No. 3,215,292 to Halls, U.S. Pat. No. 3,952,896 to Hayward and U.S. Pat. No. 4,699,560 to Ostermeyer.